1. Field of the Invention
The present invention relates to a data transmission method and a communication system using the same.
2. Description of Related Art
802.11 wireless network technique provided by the Institute of Electrical and Electronics Engineers (IEEE) has become the mainstream in the wireless network market under the strong promotion of the WiFi alliance for it is easy to install and uses only license-free bands. However, according to the original specification of 802.11 standard, all 802.11 STAs (stations) (legacy 802.11 stations) share and equally contend for the same medium. Thus, it is impossible to assign these 802.11 STAs different priorities for the contention according to the requirements of their packets to the Quality of Service (QoS). Accordingly, the IEEE designated an 802.11e team in 1999 to establish two medium access mechanisms: enhanced distributed channel access (EDCA) mechanism and hybrid coordination function (HCF) controlled channel access (HCCA) mechanism. Both of these mechanisms are backward compatible to conventional 802.11 medium access control (MAC) mechanisms, such as the distributed coordination function (DCF) and the point coordination function (PCF).
FIG. 1 illustrates the infrastructure of the EDCA mechanism, the HCCA mechanism, and the conventional 802.11 MAC mechanisms DCF and PCF. Referring to FIG. 1, the infrastructure 100 includes DCF, PCF, EDCA mechanism for HCF contention access, HCCA, and HCF, wherein these mechanisms are all within the extension of the MAC layer.
The shortcoming of DCF is that the idling time will decrease along with the increase of the contention between a plurality of workstations for a wireless medium. The shortcoming of PCF is that the polling method adopted has low efficiency. The 802.11e standard can provide better Quality of Service (QoS) to wireless networks in the MAC layer through aforementioned mechanisms.
The format of a MAC packet in the IEEE 802.11 standard is slightly changed in the IEEE 802.11e standard, wherein a two bytes traffic category identifier (TCID) field is added in the header of the MAC packet, and the last three bits in the TCID field are used for indicating the level of a workstation which transmits the packet. Totally eight levels can be distinguished by the three bits.
According to the EDCA mechanism, a priority is assigned to each transmission data, and the data having higher priority is first transmitted. The priority is related to the carrier sense time, the compensation time, and the frame transmission time. Voice data has the highest priority, and video data is next to it. It is considered that terminals are increased and accordingly the compensation time is further prolonged if a data transmission fails. According to the HCCA mechanism, priorities are assigned by a hybrid coordination program. Each terminal has to notify the hybrid coordination program in advance about the information, such as the data type, the desired band width, and the time duration thereof. Based on such information, the hybrid coordination program assigns the priorities to the terminals or performs a polling operation on all the terminals.
FIG. 2 illustrates the preset value of an EDCA parameter set, wherein AC_BE represents the optimal performance (with higher priority), AC_BK represents the background (with lower priority), AC_VI represents the video (with the highest priority), and AC_VO represents the voice (with the highest priority). In addition, CWmin represents the minimum value of a contention window, CWmax represents the maximum value of the contention window, and AIFSN represents a collision sense signal. These parameters may affect the deferring time of the access category. The values of AC_VI and AC_VO should be set to lower values if voice or video service is used, and the values of AC_BE and AC_BK should be set to higher values if E_mail or webpage service is used.
The DCF access parameters for the conventional 802.11 standard are as followings: CWmin has value aCWmin(31), CWmax has value aCWmax(1023), and DIFS is corresponding to the arbitration inter-frame space (AIFS) having AIFSN as 2. It can be observed through comparison that the medium access priority of DCF is between AC_BE and AC_VI. However, because 802.11e standard and 802.11 standard have different designs in the backoff countdown mechanism, the medium access priority of DCF is substantially corresponding to AC_BE (as shown in FIG. 3) according to the experimental result of the reference article “Understanding 802.11e contention-based prioritization mechanisms and their coexistence with legacy 802.11 stations” by G. Bianchi, I. Tinnirello, and L. Scalia in pages 28˜34, IEEE Volume 19, Issue 4, July-August 2005, Network. The abscissa in FIG. 3 indicates the number of 802.11 STAs (stations) and 802.11e QSTAs (stations with OoS) coexist in the network, and the ordinate in FIG. 3 indicates the corresponding throughput, wherein the value of CWmin is set to 31, and the value of CWmax is set to 1023. It can be observed from FIG. 3 that even though the contention windows of all the EDCA QSTAs are enlarged to the same as that of DCF, the 802.11e QSTAs have higher priorities compared to the 802.11 STAs in medium contention through the setting of AIFSN.
As shown in FIG. 2, the value of AIFSN of AC_VO and AC_VI is preset to 2. Referring to FIG. 3, it can be observed by comparing the throughputs of EDCA and DCF when the value of AIFSN is 2 that the 802.11e QSTAs have much higher chances of taking up the medium than the 802.11 STAs, and when the number of stations increases, the advantage for EDCA to contend for the medium is also multiplied and accordingly the throughput of all the DCF STAs is reduced.
Such service division could be acceptable if the packets transmitted by the 802.11 STAs are in a best effort traffic; however, if the 802.11 STA are also about to transmit voice or video packets having higher priorities, all the packets equally contend for the medium so as to obtain the transmission right with a access parameter set about the same as the best effort category because the MAC of DCF does not support access parameters of different categories, and which may cause unfairness to those packets having higher QoS requirement in the 802.11 STA. Since the packets having higher priorities of the 802.11 STA cannot obtain the access to the medium, the corresponding stations for transmitting these packets cannot provide the QoS as expected even though they are 802.11e QSTAs with QoS.
In foregoing reference article, the performance of EDCA in service differentiation is analyzed through simulative experiments, wherein it shows that the adjustment to AIFS has obviously better performance than the adjustment to the contention window in order to obtain better service differentiation performance, and meanwhile, the experimental result shows that the behaviors of a conventional 802.11 STA is very similar to that of the best effort category of an 802.11e QSTA, which also indicates the inferior performance in medium contention of the conventional 802.11 STA and the first two traffic categories having high priorities of the 802.11e QSTA in an coexistent environment thereof.
In the article “Performance Analysis of Priority Schemes for IEEE 802.11 and 802.11e Wireless LANs” by Yang Xiao, pages 1506˜1515, in Wireless Communications, IEEE Transactions on Volume 4, Issue 4, July 2005, an analysis model adaptive to EDCA is provided, wherein it shows that the performance of EDCA in its throughput and QoS (including delay and packet loss rate) is closely related to the settings of its parameters.
In the article “Impact of frame size, number of stations and mobility on the throughput performance of IEEE 802.11e” by J. del Prado Pavon, S. N. Shankar in pages 789˜795 of Wireless Communications and Networking Conference, 2004. WCNC. 2004 IEEE Volume 2, 21-25 Mar. 2004 Vol. 2, the effect of frame size to EDCA is explained with experimental results, and the experiment shows that the throughput performance of EDCA is less affected by bad link compared to the conventional DCF.
In some other articles, an admission control algorithm is provided regarding EDCA, wherein the QoS of packets having high priorities are maintained by managing the traffic of the entire network, or a mechanism regarding HCCA environment is provided, wherein the value of TXOP is set according to the actual transmission rate of packets in the physical layer so as to provide more efficient QoS, or the problem of radio resource sharing caused by QoS supporting basic service set (QBSS) working in the same band regarding EDCA environment is resolved.